Production of Graphene

ABSTRACT

An apparatus for large-scale production of graphene and graphene oxide is provided. The apparatus includes a first electrode, a second electrode, an electrobath, a power supply, and a module for filtering and separating the graphene products. Large amounts of graphene and graphene oxide can be produced rapidly using electrochemical exfoliation. High-quality graphene and graphene oxide can be produced under the room temperature in a simple and cost-effective way.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Taiwan application 100148453, filedon Dec. 23, 2011, the content of which is incorporated by reference.

TECHNICAL FIELD

The disclosure relates to production of graphene.

BACKGROUND

A graphene sheet is composed of carbon atoms occupying a two-dimensionalhexagonal lattice. Graphene can have a high carrier mobility andexcellent thermal conductivity. Graphene can be used in, e.g.,semiconductor devices, touch panels, and solar cells. Graphene can befabricated by, e.g., mechanical exfoliation, epitaxial growth, chemicalvapor deposition, and chemical exfoliation. Methods for producinggraphene have been described in U.S. Pat. No. 7,790,285, U.S. Pat. No.7,892,514, and U.S. patent application Ser. No. 13/170,624, filed onJun. 28, 2011.

SUMMARY

In one aspect, an apparatus is provided for preparing high-qualitygraphene and graphene oxide in a simple and fast manner at a low cost.The graphene and graphene oxide can be produced by electrochemicalexfoliation.

In one aspect, an apparatus for producing graphene and/or graphene oxideis provided. The apparatus includes a first electrode that includesgraphite; a second electrode; a container that contains an electrolyte,in which the first and second electrodes are immersed in theelectrolyte; a power supply to supply bias voltages across the first andsecond electrodes to cause intercalation of graphite and exfoliation ofgraphene; and a filtration module to separate the graphene fromun-exfoliated graphite particles and collect the graphene.

In one aspect, a method for producing graphene and/or graphene oxideincludes immersing a first electrode and a second electrode in anelectrolyte, the first electrode including graphite; applying a firstvoltage across the first and second electrodes to cause intercalation ofthe graphite to form a graphite intercalation compound; applying asecond voltage across the first and second electrodes to exfoliate thegraphite intercalation compound to produce at least one of graphene orgraphene oxide; filtering the electrolyte using a first filter thatblocks un-exfoliated graphite particles and allows the at least one ofgraphene or graphene oxide to pass through; and filtering theelectrolyte using a second filter to collect the at least one ofgraphene or graphene oxide.

In one aspect, an apparatus for producing graphene and graphene oxideincludes a first electrode that has an electrode holder having astarting graphite material, a second electrode, an electrobath, a powersupply, and a module for filtering and separating the graphene products.

Implementations of the apparatus can include one or more of thefollowing features. The first electrode can be an electrode holder thatincludes the starting graphite material, and the second electrode can beeither an electrode holder that includes the starting graphite materialor a metal. The starting graphite material can include a mixture ofgraphite and metal. To electrochemically exfoliate graphene, the firstelectrode and the second electrode can be placed in an electrolyte.Intercalation of the graphite material is performed using a first biasvoltage, and the exfoliation of the graphite material is performed usinga second bias voltage. The solid graphene products are taken out of theelectrolyte. The final graphene product is not necessarily limited tobeing dissolved in the electrolyte. In some examples, the final grapheneproduct can also be collected in an electrode holder.

The starting graphite material can include natural graphite in a layeredstructure, artificial graphite, composite material prepared fromgraphite powder, or a combination of the above. The starting graphitematerial can include natural graphite, highly-oriented pyrolyticgraphite (HOPG), pitch-based graphite, resin-based graphite, PAN-basedcarbon fibers, pitch-based carbon fibers, coal, a carbon materialcontaining graphite layers, and/or a carbon material containing graphiteflakes. The starting graphite material can be a crystalline graphitelayer material in the form of large particles, fragments, or powder, orhaving an irregular shape. The starting graphite material can also be ablock of graphite material made of pieces having the forms describedabove and held together by an electrically conductive adhesive.

To increase the efficiency of mass production of graphene, each of thetwo electrodes can include two or more sub-electrodes connected inparallel or in an array. Each sub-electrode can include a startinggraphite material or an electrode holder that includes the startinggraphite material.

The metal in the second electrode can be a precious metal that isresistant to acid and alkaline, such as platinum (Pt), silver (Ag), gold(Au), iridium (Ir), osmium (Os), palladium (Pd), rhodium (Rh) orruthenium (Ru). Other electrically conducting material that is resistantto chemical etching, such as copper (Cu), stainless steel, graphite,glassy carbon, or conducting polymer, can also be used in the secondelectrode.

The electrolyte can be placed in a container made from glass, polymermaterial (e.g., acrylic, polypropylene, polystyrene, or polyvinylchloride), stainless steel, or another metal material. The electrolytecan include hydrogen bromide, hydrochloric acid, or sulfuric acid.Alkaline such as potassium hydroxide or sodium hydroxide can be added tothe electrolyte. The electrolyte can include an oxidant, which caninclude potassium bichromate, permanganic acid or potassiumpermanganate.

To enhance the graphene or graphene oxide production efficiency and toenhance the graphene product quality, the electrolysis process may besupplemented with heating, application of ultrasound or microwave,application of high-energy light radiation, and/or stirring theelectrolyte by a rotor as needed to facilitate exfoliating of grapheneor graphene oxide.

To implement a continuous production process, the exfoliated productscan be fed into a module for filtering and separating the grapheneproducts. The first component of the module is a microporous sievehaving a size of about 18 mesh to 1250 mesh, preferably from 35 mesh to500 mesh, for filtering large graphite particles that have not beenexfoliated. The microporous sieve allows products having the desiredsizes to pass through. The second component of the module is afiltration membrane that collects the graphene sheet product that passedthrough the sieve. The membrane can have a pore diameter of about 200 nmto 10 μm, preferably from about 500 nm to 5 μm. The graphene productcollected from the filtration membrane can be treated with deionizedwater or other ionic solutions (such as hydrochloric acid) capable ofdissolving or replacing the residual ions (such as potassium ion orsulfite ion) to remove the residual electrolyte.

The bias voltage can be provided by a direct current (DC) or alternatingcurrent (AC) power supply. The power supply can be controlled in aconstant voltage mode or a constant current mode. In the examples below,the power supply operates in a constant voltage mode to providecontrolled bias voltages. The first bias voltage can range from 0.5 V to10 V, preferably from 2.5 V to 5 V. The second bias voltage can rangefrom 5 V to 220 V, preferably from 10 V to 100 V.

In one aspect, a method is provided for mass production of graphene andgraphene oxide by using the apparatus described above that useselectrochemical exfoliation in which intercalation and exfoliation ofgraphite material is performed at room temperature through changingvoltage without applying high temperature for reduction. The process issimplified and a large amount of highly graphitized graphene with a fewlayers and large lateral dimensions can be prepared within a shortperiod of time.

In one aspect, a method is provided for mass production of graphene andgraphene oxide. The method includes placing the first electrode and thesecond electrode in an electrolyte, in which the first electrode is anelectrode holder that includes a starting graphite material, and thesecond electrode is an electrode holder that includes a startinggraphite material or a metal. The starting graphite material can includea mixture of graphite and metal. The intercalation of the startinggraphite material is performed under a first bias voltage, and theexfoliation of the starting graphite material is performed under asecond bias voltage. The exfoliated products are fed into a module forfiltering and separating the products. The module includes a firstcomponent, which can be a microporous sieve, and a second component,which can be a filtration membrane. The graphene product and/or grapheneoxide are collected from the filtration membrane.

Implementations of the method can include one or more of the followingfeatures. The first electrode and the second electrode can be used asthe anode and the cathode, respectively, and can be wrapped with anelectrode wire and immersed in an electrolyte. The graphite material canbe subject to the intercalation step under the first bias voltage inwhich the anions in the electrolyte, such as sulfate ions and nitrateions, are intercalated into the interlayer space between two adjacentgraphite layers or their grain boundary due to the electric fieldproduced by the bias voltage. The first bias voltage can range from 0.5V to 10 V, preferably from 2.5 V to 5 V, and the reaction time can rangefrom about 1 minute to 30 minutes, preferably from 1 minute to 5minutes.

Subsequently, the graphite material is subject to the exfoliation stepusing the second bias voltage, in which the second bias voltage isgreater than the first bias voltage and can range from 5 V to 220 V,preferably from 10 V to 100 V, and there is no restriction on thereaction time.

The method can further include performing the exfoliation step for thegraphite material using a third bias voltage that is different from thesecond bias voltage. For example, the third bias voltage and the secondbias voltage may have opposite polarities or have the same polarity butdifferent values, in which the second bias voltage and the third biasvoltage may be direct current and alternating current, respectively. Ifthe third bias voltage and the second bias voltage have oppositepolarities, then the positive voltage can facilitate exfoliating andoxidizing the graphene and subsequently the negative voltage can reducethe oxidized graphene. For example, the second bias voltage can be 10 Vand the third bias voltage can be −10 V, in which the reaction time is 2seconds and 5 seconds, respectively, and the two voltages arealternatively applied for a certain period of time. The intercalationstep and the exfoliation step can be performed by switching betweendifferent first and second bias voltages, depending on the compositionand acidity of the electrolyte to achieve the optimal quality and yield.

In some implementations, a first bias voltage (e.g., 0.5V) can beapplied to the electrodes for a period of time to cause intercalation,and a second bias voltage (e.g., 5V) can be applied to the electrodes tocause exfoliation. In some implementations, during the exfoliation step,a second bias voltage (e.g., 5V) and a third bias voltage (e.g., −5V)are alternately applied to the electrodes, each of the second and thirdbias voltages being applied for a certain period of time (e.g., 2seconds). The graphene obtained using the second method of alternatingbetween the second and third bias voltages may have a higher quality,compared to using only the second bias voltage.

The bias voltage can be selected based on a desired production rate andquality of products. For example, when the absolute value of the biasvoltage is higher, the exfoliation rate may be faster but the quality ofthe produced graphene may be lower. When the absolute value of the biasvoltage is lower, the exfoliation rate is slower but the graphenequality may be better.

After the exfoliation step, the exfoliated products may be provided to amodule for filtering and separating the products. The first component ofthe module is a microporous sieve (e.g., 35 mesh) for filteringun-exfoliated large graphite particles and obtaining the products insuitable sizes that pass through the sieve. The second component of themodule is a filtration membrane having a pore diameter of, e.g., 400 nmfor collecting the graphene sheet product passing through the sieve. Thegraphene product and graphene oxide collected from the second componentcan be treated with a large amount of deionized water to remove theresidual electrolyte, or be treated with other ionic solutions (such ashydrochloric acid) capable of dissolving or replacing the residual ion(such as potassium ion or sulfite ion) to remove the residualelectrolyte. The module mainly relies on an air pump to accelerate thefiltration process. In some examples, filtration and separation can beachieved by vacuum filtration.

Advantages of the aspects, systems, and methods may include one or moreof the following. High quality graphene and graphene oxide can beproduced in mass quantities. The cost for producing the graphene andgraphene oxide is low. During the production process, it is notnecessary to subject the graphene or graphene oxide to a hightemperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of an apparatus for producing graphene and grapheneoxide.

FIG. 2 is a diagram of an exemplary electrode holder that includesstarting graphite material.

FIG. 3 is a photo showing graphene dispersed in 250 mL of DMF.

FIG. 4 is a photo showing solid graphene after separation andfiltration.

FIG. 5 is a scanning electron microscope image of exemplary solidgraphene powder.

FIG. 6A is an image of an exemplary graphene sheet.

FIG. 6B is a graph showing exemplary measurements by an atomic forcemicroscope of a portion of the graphene sheet shown in FIG. 6A.

FIG. 7 is a graph shows data obtained from a confocal Raman microscopicsystem for analyzing the bonding of graphene sheets.

FIG. 8 is a graph showing the characteristic Raman peaks of grapheneoxide.

FIG. 9 is a diagram of an apparatus for producing graphene and grapheneoxide.

FIG. 10 is a flowchart of an exemplary procedure for producing graphene.

DETAILED DESCRIPTION

Graphene sheets can be mass produced by a process that includesintercalation, exfoliation, and filtration. The intercalation andexfoliation are performed in an electrolyte. Graphene and graphene oxideare separated from un-exfoliated graphite particles by a filter having amesh size selected to block the un-exfoliated graphite particles andallow the graphene and graphene oxide to pass. The graphene and grapheneoxide are collected by a filtration membrane having a pore size smallerthan the sizes of the graphene and graphene oxide to be collected.

Referring to FIG. 1, an exemplary graphene production system 100includes a first electrode 102 and a second electrode 104. In thisexample, the first electrode 102 is an anode and the second electrode104 is a cathode. Each of the electrodes 102 and 104 is wrapped with anelectrode wire (not shown) and immersed in an electrolyte 106. The firstelectrode 102 can be made of a starting graphite material or include aholder that contains starting graphite material. The second electrode104 can be made of a starting graphite material or metal, or include aholder that contains a starting graphite material. For example, thestarting graphite material can include highly-oriented pyrolyticgraphite (HOPG), pitch-based graphite, resin-based graphite,polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, orcoal. The metal used for the second electrode 104 can be a preciousmetal that is resistant to chemical etching, such as platinum, silver,gold, iridium, osmium, palladium, rhodium, or ruthenium. The secondelectrode 104 can also be made of other conducting materials, e.g.,copper, stainless steel, glassy carbon, or conducting polymer.

The graphene production system 100 can also be used to produce grapheneoxides.

Each of the first electrode 102 and the second electrode 104 can be madeof crystalline graphite layer material in the form of large particles,flakes, or powder, or having an irregular shape. The electrodes can alsobe a bulk material composed of graphite particles, flakes, or powderbonded together by an electrically conductive adhesive. In someimplementations, to increase throughput in mass production, an electrodecan include two or more sub-electrodes connected in parallel or in anarray.

In some implementations, the electrolyte 106 can be placed in acontainer 108 made from glass, polymer material (e.g., acrylic,polypropylene, polystyrene, or polyvinyl chloride), stainless steel, orother metals. The electrolyte can include hydrogen bromide, hydrochloricacid, or sulfuric acid. Potassium hydroxide can be added to theelectrolyte. The electrolyte can include an oxidant, such as potassiumbichromate, permanganic acid or potassium permanganate.

A voltage supply 114 provides a bias voltage to the first and secondelectrodes 102 and 104. The voltage supply 114 can supply adirect-current (DC) or alternating-current (AC) bias voltage. When afirst bias voltage (e.g., in a range from 0.5 V to 10 V) is applied tothe first and second electrodes 102 and 104, ions in the electrolyte 106can penetrate spaces between layers in the starting graphite material toform a graphite intercalation compound. This is referred to as theintercalation step. When a second bias voltage (e.g., in a range from 5V to 220 V) is applied to the first and second electrodes 102 and 104,the graphite intercalation compound is exfoliated to form graphenesheets that are dispersed in the electrolyte 106.

The graphene sheets are can be removed from the electrolyte using afiltration process. A pump 118 pumps the electrolyte 106, which containsthe graphene sheets, to the filtration module 116. The filtration module116 includes a microporous sieve 120 and a filtration membrane 122. Themicroporous sieve 120 filters out un-exfoliated large graphite particlesand allows graphene sheets of suitable sizes to pass. For example, themicroporous sieve 120 can have a size in a range from 18 mesh to 1250mesh, preferably 35 mesh. The size of the sieve can be selecteddepending on the size of the exfoliated products. The filtrationmembrane 122 collects the graphene sheets that passed through the sieve118. The filtration membrane 122 can have a pore diameter in a rangefrom 200 nm to 1,200 nm, preferably 400 nm. The size of the porediameter can be selected depending on the size of the exfoliatedproducts. The graphene sheets collected at the filtration membrane 122can be washed with a large amount of deionized water to remove theresidual electrolyte. The graphene sheets can also be washed by usingother ionic solutions (such as hydrochloric acid) capable of dissolvingor replacing the residual ion (such as potassium ion or sulfite ion) toremove the residual electrolyte. In some examples, the pump 118 isstopped periodically, the filtration membrane 122 is removed from thefiltration module 116 and rinsed. The graphene is removed from thefiltration membrane 122. An air pump 124 provides an additional suctionforce to accelerate the filtration process. The electrolyte can berecycled after being filtered by the filtration module 116.

The efficiency of exfoliation can be enhanced by heating the electrolyte106 using a temperature controller 112 and/or stirring the electrolyte106 using a rotor 110. The efficiency of exfoliation can also beenhanced by applying ultrasound microwave to the electrolyte 106 and/orirradiating the electrolyte 106 with high-energy light.

If the starting graphite material is in a block form having an edgelength of, e.g., 1 centimeter or more, the starting graphite materialmay be directly connected to the voltage supply 114 and be used as anelectrode. Referring to FIG. 2, if the starting graphite material isfragmented or in powder form, an electrode holder 140 can be used aspart of the first electrode 102 and/or the second electrode 104. Theelectrode holder 140 includes a separation sieve 142 that holds astarting graphite material 144, which may be crystalline graphite layermaterial that is fragmented or in powder form, or other formats thatprevent direct connection to the voltage supply 114. The separationsieve 142 can be made of, e.g., a filter sieve (e.g., having a pore sizein a range from 0.1 mm to 5 mm), a porous membrane, or a component withpores.

The separation sieve 142 can have one or more of the followingfunctions. The separation sieve 142 can allow dispersed startinggraphite material to maintain a compact and conducting state and beelectrically connected to the voltage supply 114. The separation sieve142 can allow the starting graphite material to access the electrolyte106 to effect electrochemical exfoliation. The separation sieve 142 canallow exfoliated graphene sheets to diffuse into the electrolyte 106 sothat remaining unreacted starting graphite material can continue toreact with the electrolyte. The separation sieve 142 can have a poresize that is related to the dimensions of the starting graphite materialand should be configured to prevent the unreacted starting graphitematerial from passing through.

In some examples, the separation sieve 142 can have a pore size selectedto be sufficiently small to trap the produced graphene sheets inside theseparation sieve 142. This way, the graphene sheets can be collected inthe electrode holder 140.

The separation sieve 142 can be made of an electrically insulatingmaterial that is resistant to acid and alkaline. For example, theseparation sieve 142 can be made of glass, acrylic, polypropylene,polystyrene, polyvinyl chloride, other polymer materials, or metalmaterials that have been processed by insulation and anti-corrosiontreatments. A metal electrode 146 electrically connects the startinggraphite material to an external circuit that is electrically connectedto the voltage supply 114. The metal electrode 146 applies a fixedpressure to the starting graphite material 144 so that the graphitefragments or powder are coupled more closely together to improve betterconductance.

FIG. 3 is a photo 150 of a solution obtained using the grapheneproduction system 100 (FIG. 1). The solution includes graphene sheetsthat are dispersed in 250 mL of dimethylformamide (DMF). The solution issometimes referred to as a “graphene ink” and can be used to producegraphene thin films.

FIG. 4 is a photo 160 showing solid graphene powder obtained using thegraphene production system 100.

The graphene sheets were observed by using a scanning electronmicroscope (SEM), model JEOL-6330F, and an atomic force microscope(AFM), Veeco Dimension-Icon system. FIG. 5 is a scanning electronmicroscope image 170 of the solid graphene sheet powder obtained usingthe graphene production system 100. The graphene powder includesgraphene sheets stacked in layers and has a high purity.

FIG. 6A is an image 180 of a graphene sheet 182 that was produced bydroplet plating graphene ink (made using the graphene production system100) onto a silica (SiO₂) substrate. The graphene sheet 182 was observedusing an atomic force microscope.

FIG. 6B is a graph 190 showing measurements made by an atomic forcemicroscope along a line 184 on the graphene sheet 182. A curve 192indicates that the thickness of the graphene sheet 182 is not greaterthan 3 nm. Additional measurements indicate that about 65% of thegraphene sheet has a thickness of less than 2 nm.

FIG. 7 is a graph 200 shows data obtained from a NT-MDT confocal Ramanmicroscopic system for analyzing the bonding of graphene sheets. In thisexample, a 1.6-nm thick graphene sheet (based on measurements from anatomic force microscope) was excited at a wavelength of 473 nm by theNT-MDT confocal Raman microscopic system, and the molecular bondingstructure of the graphene sheet was analyzed. A curve 202 indicates thata G peak 204 at around 1580 cm⁻¹ is narrow and shows a high intensity,indicating that the graphene obtained according to the productionprocess described above has excellent graphitization. In general, the2D/G intensity ratio of a single-layer graphene is greater than the 2D/Gintensity ratio of a double-layer graphene, and the intensity ratio of a2D peak 206 at around 2720 cm⁻¹ to the G peak 204 is greater than thatof the single-layer reduced graphene oxide produced using conventionalmethods. This shows the graphene produced by the graphene productionsystem 100 has excellent graphitization.

The graphene production system 100 can also be used to produce grapheneoxide by using a process similar to that for producing graphenedescribed above, but with increased DC bias voltage for electrolysis orincreased acidity for the electrolyte. In some examples, applying a biasvoltage having a higher absolute value and using an electrolyte having ahigher acidity level tend to produce more graphene oxide (compared toapplying a bias voltage having a lower absolute value and using anelectrolyte having a lower acidity level). The graphene oxide can bepurified by filtration and rinsing by water. FIG. 8 is a graph showingthe characteristic Raman peaks of graphene oxide prepared by using thismethod.

Referring to FIG. 9, an exemplary graphene production system 220includes a controller 222 that automatically controls the bias voltagesapplied to the first and second electrodes 102, 104. The grapheneproduction system 220 includes other components similar to those of thegraphene production system 100 (FIG. 1).

For example, the controller 222 can have a user interface (not shown)that allows a user to select pre-stored modes of operation. Thecontroller 222 can have a first mode of operation in which a first biasvoltage is applied to the electrodes 102, 104 for a period of time tocause intercalation and a second bias voltage is applied to theelectrodes 102, 104 to cause exfoliation. The controller 222 can have asecond mode of operation in which a first bias voltage is applied to theelectrodes 102, 104 for a period of time to cause intercalation, and asecond bias voltage and a third bias voltage are alternately applied tothe electrodes 102, 104 to cause exfoliation. The controller 222 mayallow the user to choose between a first mode for producing higherthroughput but lower quality graphene, or a second mode for producinglower throughput but higher quality graphene. The controller 222 may beprogrammable such that the user can set a sequence of voltages levels tobe applied to the electrodes 102, 104 over time.

The graphene production system 220 may include sensors 224 that detectthe amount of graphene being collected on the filtration membrane 122.The sensor signals are sent to the controller 222. The controller 222may stop the pump 118 and initiate a process for collecting thegraphene. For example, the controller 222 may control a robotic arm (notshown) to retrieve the filtration membrane 122, wash the filtrationmembrane 122 with deionized water, scrape the graphene off thefiltration membrane 122, collect the graphene in a container, wash thefiltration membrane 122 again, and place the filtration membrane 122back in the filtration module 116. The controller 222 may start the pump118 so that the filtration membrane 122 can continue to collect graphenesheets.

The graphene production system 220 may determine the amount of grapheneproduced per unit of time, and adjust the voltage applied to theelectrodes 102, 104 to adjust the production rate. For example, theamount of graphene recovered from the filtration membrane 122 can bedivided by the amount of time used for collecting the graphene togenerate an estimate of the production rate. The graphene may beautomatically analyzed to determine its quality, and the qualityinformation is provided to the controller 222, which in turn adjusts thebias voltage level to adjust the quality of the graphene. By usingvarious sensors to detect the production throughput and the graphenequality and sending the sensor information to the controller 222, thegraphene production system 220 can control the production throughput andthe quality of the graphene to meet preset requirements.

The controller 222 may include a programmable system having at least oneprogrammable processor coupled to receive data and instructions from,and to transmit data and instructions to, a data storage system forstoring data and instructions. The at least one programmable processorcan include, e.g., general purpose microprocessors, special purposemicroprocessors, or digital signal processors.

FIG. 10 is a flowchart of an exemplary procedure 230 for producinggraphene. The procedure 230 may be performed by, e.g., the grapheneproduction system 100 (FIG. 1) or the graphene production system 220(FIG. 9).

The procedure 230 includes immersing (232) a first electrode and asecond electrode in an electrolyte, the first electrode includinggraphite. For example, the first electrode can include natural graphite,highly-oriented pyrolytic graphite, pitch-based graphite, carbon fibers,coal, a material including graphite layers, or a material comprisinggraphite flakes. The first electrode can include two sub-electrodesconnected in parallel. The second electrode can include a holder thatholds graphite or a mixture of graphite and metal. The metal can includea precious metal that is resistant to acid. The second electrode caninclude two sub-electrodes connected in parallel. The electrolyte caninclude hydrogen bromide, hydrochloric acid, or sulfuric acid.

A first bias voltage is applied (234) across the first and secondelectrodes to cause intercalation of the graphite to form a graphiteintercalation compound. For example, the first bias voltage can be in arange from 0.5 V to 10 V. The first bias voltage can be a DC voltage oran AC voltage.

A second voltage is applied (236) across the first and second electrodesto exfoliate the graphite intercalation compound to produce graphenesheets. For example, the second bias voltage can be in a range from 5 Vto 220 V. The second bias voltage can be a DC voltage or an AC voltage.

The electrolyte is filtered (238) using a first filter that blocksun-exfoliated graphite particles and allows graphene sheets to passthrough. For example, the first filter can include a microporous sieve.The microporous sieve can have a size in a range from 18 mesh to 1,250mesh.

The electrolyte is filtered (240) using a second filter to collect thegraphene sheets. The second filter can include a filtration membrane.The filtration membrane can have a pore diameter in a range from 200 nmto 1,200 nm.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments.

A number of implementations of the invention have been described.Nevertheless, it will be understood that various modifications can bemade without departing from the spirit and scope of the invention. Insome cases, the actions recited in the claims can be performed in adifferent order and still achieve desirable results. In addition, theprocesses depicted in the accompanying figures do not necessarilyrequire the particular order shown, or sequential order, to achievedesirable results. Other implementations and applications are alsowithin the scope of the following claims.

What is claimed is:
 1. An apparatus for producing at least one ofgraphene or graphene oxide, the apparatus comprising: a first electrodethat includes graphite; a second electrode; a container that contains anelectrolyte, in which the first and second electrodes are immersed inthe electrolyte; a power supply to supply bias voltages across the firstand second electrodes to cause intercalation of graphite and exfoliationof graphene; and a filtration module to separate the graphene fromun-exfoliated graphite particles and collect the graphene.
 2. Theapparatus of claim 1 in which the first electrode comprises an electrodeholder that holds graphite.
 3. The apparatus of claim 2 in which thesecond electrode comprises an electrode holder that holds graphite. 4.The apparatus of claim 2 in which the electrode holder comprises aseparation sieve having a pore size selected to allow the electrolyte topass but prevent un-exfoliated graphite from passing.
 5. The apparatusof claim 4 in which the separation sieve has a pore size selected toprevent a portion of the graphene from passing.
 6. The apparatus ofclaim 1 in which the second electrode is made of metal.
 7. The apparatusof claim 6 in which the metal comprises a precious metal that isresistant to acid.
 8. The apparatus of claim 1 in which the secondelectrode is made of a mixture of graphite and a metal.
 9. The apparatusof claim 1 in which the first electrode comprises at least one ofnatural graphite, highly-oriented pyrolytic graphite, pitch-basedgraphite, carbon fiber, coal, a material comprising graphite layers, ora material comprising graphite flakes.
 10. The apparatus of claim 1 inwhich the first electrode comprises two sub-electrodes connected inparallel.
 11. The apparatus of claim 10 in which the second electrodecomprises two sub-electrodes connected in parallel.
 12. The apparatus ofclaim 1 in which the electrolyte comprises at least one of hydrogenbromide, hydrochloric acid, or sulfuric acid.
 13. The apparatus of claim1, comprising an air pump to pump the electrolyte through the filtrationmodule.
 14. The apparatus of claim 1 in which the filtration modulecomprises a microporous sieve and a filtration membrane.
 15. Theapparatus of claim 14 in which the microporous sieve has a size in arange from 18 mesh to 1250 mesh.
 16. The apparatus of claim 14 in whichthe filtration membrane has a pore diameter in a range from 200 nm to1,200 nm.
 17. The apparatus of claim 1, comprising a controller tocontrol the power supply to provide a first bias voltage to causeintercalation and a second bias voltage to cause exfoliation.
 18. Theapparatus of claim 1, comprising a controller to control the powersupply to provide a first bias voltage during an intercalation step, andalternately provide a second bias voltage and a third bias voltageduring an exfoliation step.
 19. The apparatus of claim 18 in which thesecond bias voltage and the third bias voltage have opposite polarities.20. A method for producing graphene, comprising: immersing a firstelectrode and a second electrode in an electrolyte, the first electrodecomprising graphite; applying a first voltage across the first andsecond electrodes to cause intercalation of the graphite to form agraphite intercalation compound; applying a second voltage across thefirst and second electrodes to exfoliate the graphite intercalationcompound to produce at least one of graphene or graphene oxide;filtering the electrolyte using a first filter that blocks un-exfoliatedgraphite particles and allows the at least one of graphene or grapheneoxide to pass through; and filtering the electrolyte using a secondfilter to collect the at least one of graphene or graphene oxide. 21.The method of claim 20 in which the first filter comprises a microporoussieve.
 22. The method of claim 20 in which the second filter comprises afiltration membrane.
 23. The method of claim 20 in which the secondelectrode comprises a holder that holds graphite or a mixture ofgraphite and metal.
 24. The method of claim 20 in which the firstelectrode comprises at least one of natural graphite, highly-orientedpyrolytic graphite, pitch-based graphite, carbon fibers, coal, amaterial comprising graphite layers, or a material comprising graphiteflakes.
 25. The method of claim 20 in which the first electrodecomprises two sub-electrodes connected in parallel.
 26. The method ofclaim 25 in which the second electrode comprises two sub-electrodesconnected in parallel.
 27. The method of claim 20 in which the metalcomprises a precious metal that is resistant to acid.
 28. The method ofclaim 20 in which the electrolyte comprises at least one of hydrogenbromide, hydrochloric acid, or sulfuric acid.
 29. The method of claim20, comprising an air pump to pump the electrolyte and cause theelectrolyte to pass the first and second filters.
 30. The method ofclaim 20 in which the first voltage is in a range from 0.5 V to 10 V.31. The method of claim 20 in which the second voltage is in a rangefrom 5 V to 220 V.
 32. The method of claim 20 in which the voltagecomprises a DC voltage.
 33. The method of claim 20 in which the voltagecomprises an AC voltage.
 34. The method of claim 20 in which the firstfilter comprises a microporous sieve having a size in a range from 18mesh to 1,250 mesh.
 35. The method of claim 20 in which the secondfilter comprises a filtration membrane having a pore diameter in a rangefrom 200 nm to 1,200 nm.
 36. The method of claim 20, comprisingproviding a third voltage during an exfoliation step, the third voltagehaving a polarity that is opposite to that of the second voltage. 37.The method of claim 36, comprising during the exfoliation step,alternately providing the second voltage and the third voltage acrossthe first and second electrodes.